Method and system for providing content to a recipient device

ABSTRACT

The invention relates to a computer-implemented method for providing content to a particular recipient device of a plurality of recipient devices. Copies of one or more content elements of the content are generated and one or more of the copies are modified to obtain modified copies of the content elements. The content elements, including the one or more modified copies of the content elements, are stored in a storage. Selection information is transmitted to the particular recipient device in response to a request for providing the content. The selection information prescribes to the recipient device the modified copy to be retrieved by the recipient device for substantially each content element for which a modified copy is available.

FIELD OF THE INVENTION

Generally, the invention relates to the field of content delivery torecipient devices. More specifically, the invention relates to the fieldof streaming-type content delivery to a particular recipient device.

BACKGROUND OF THE INVENTION

The latest developments of internet streaming protocols for deliveringmultimedia content brought up protocols that allow transmitting contentas a set of time-bounded content elements, also referred to as chunks,that are physically separated (e.g. a file is created for each chunk ofcontent) or logically separated (e.g. all chunks of content are storedin a single file with an addressing structure that allows to access anychunk individually). The former technique is used by HTTP

Live Streaming or 3GPP adaptive HTTP Streaming, whereas the lattertechnique is employed by Microsoft Smooth Streaming. The Internetstreaming protocols allow a recipient device to download a descriptionfile (also known as “manifest” or “play-list”) that, among other things,lists location(s) of content elements and describes rules for forming arequest to access a content element. To consume content, the recipientdevice first obtains the description file and then obtains and consumeseach content element indicated in the description file.

When multiple recipient devices request the same content element fromthe server, there is a high probability that some of these recipientdevices will receive the content element from the cache of anintermediate network node, instead of from the server itself. Receivingcontent elements from intermediate network node caches provides forscalability of the content delivery network, reduces the load for thestreaming server and decreases delivery latency.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and system thatallow tracing of and/or protection against illegal copying of content instreaming content delivery networks.

One aspect of the invention discloses a computer-implemented method forproviding content to a particular recipient device of a plurality ofrecipient devices. Copies of one or more content elements of the contentare generated and one or more of the copies are modified to obtainmodified copies of the content elements. The content elements, includingthe one or more modified copies of the content elements, are stored in astorage. Selection information is transmitted to the particularrecipient device in response to a request for providing the content. Theselection information prescribes to the recipient device the modifiedcopy to be retrieved by the recipient device for substantially eachcontent element for which a modified copy is available.

In another aspect of the invention, a system for providing content to aparticular recipient device of a plurality of recipient devices over anetwork is disclosed. The system comprises a copy generator configuredfor generating copies of one or more content elements of the content anda modifying module configured for modifying one or more of the copies ofthe content elements to obtain modified copies of the content elements.The system further comprises a processor and a transmitter and hasaccess to a storage. The storage is configured for storing the contentelements, including the one or more modified copies of the contentelements. The processor is configured for generating selectioninformation prescribing to the recipient device the modified copy to beretrieved by the recipient device for substantially each content elementfor which a modified copy is available. The transmitter is configuredfor transmitting the selection information to the particular recipientdevice in response to a request from the recipient device.

By producing several copies of a content element, each of the copiesbeing modified differently, a data structure of selection informationcan be created that instructs the recipient device which copy for aparticular content element should be retrieved for accessing andrendering the content. Modified copies of a content element may e.g.relate to different watermarks (claim 2) used for (copies of) thecontent element or to different encryption steps (claim 3) applied for(copies of) the content element. The selection information enablescustomization of the sequences of content elements for the particularrecipient device as the selection information varies for differentrecipient devices and each recipient device is enforced to retrieve onlythe prescribed copies. By applying the modification to individualcontent elements, modified copies of the content elements can be storedin one or more caches of intermediate network nodes in the deliverynetwork. The modified copies can be retrieved from these caches, insteadof from the server, when a modified content element reference is commonto the selection information of multiple recipient devices as may occurfrequently.

The embodiments of the invention as defined in claims 2 and 9 allow thesequence of content elements to be individualized for a particularrecipient device or session with the device by selecting differentparticular copies with watermarks to obtain a unique combination ofwatermarks for that device or session. Watermarking involves theinsertion of unique information into the content elements in anon-removable manner and in a manner typically not or hardly noticeablefor the user at the receiving side. The unique combination of watermarksenables tracing a recipient device redistributing the content withoutauthorization.

The embodiments of the invention as defined in claims 3 and 10 providefor a different type of modification, viz. encryption. The embodimentallows content encryption while maintaining the advantage of caching ofcontent elements in a delivery network. Encryption can be combined withwatermarking, if required.

The embodiments of the invention as defined in claims 4 and 11 enableapplication of the invention for adaptive streaming. Adaptive streamingprotocols extend the basic content streaming protocols with support forthe delivery of content over a network infrastructure that has noQuality-of-Service guarantees. Copies of the same content elements withdifferent encoding characteristics are generated. Examples of differentencoding characteristics include different bit-rates and/or differentspatial resolution. Copies with different encoding characteristics arealso referred to as quality levels. Since each quality level (copy)consists of a sequence of time aligned content elements, a recipientdevice determines to request such a content element from one of severalquality levels at run-time to react to varying network transmissionand/or processing conditions.

It should be appreciated that copies of content elements may begenerated either before or after generating content elements.

The embodiments of the invention as defined in claims 5 and 12 providefor different methods of storing the content elements. In oneembodiment, content elements are stored as individual files inaccordance with e.g. 3GPP adaptive HTTP Streaming or HTTP LiveStreaming. Further information on the latter protocol can e.g. beobtained from Roger Pantos ed, “HTTP Live Streaming” 2009, available athttp://tools.ietf.org/id/draft-pantos-http-live-streaming-00.txt whichis incorporated in the present application by reference. Alternatively,the content elements may be contained in a single file in an addressablemanner in accordance with e.g. Microsoft SmoothStreaming as known e.g.from Microsoft document “IIS Smooth Streaming Transport Protocol”, March2009 by A. Zambelli, available athttp://www.microsoft.com/downloads/en/details.aspx?displaylang=en&FamilyID=03d22583-3ed6-44da-8464-b1b4b5ca7520.

The embodiment of the invention as defined in claim 6 defines that thenumber of requests for content elements issued by the recipient devicesfor retrieving the content is less than the number of requests forcontent elements received at the server, thereby expressing that anumber of requests is fulfilled from caches at intermediate networknodes.

The embodiments of the invention as defined in claims 7 and 13 avoidstructured naming of the (modified copies of the) content elements andcomplicates making educated guesses for references to differentlymodified copies by attackers attempting to circumvent e.g. awatermarking tracking mechanism. The destructuring can be implemented invarious manners, e.g. by randomizing the references or by applying ahash function to the references.

Still other aspects of the invention relate to a, preferablynon-transitory, computer program for performing the method of one ormore of the claims 1-7, to a data structure representing the selectioninformation as described herein and to a client device configured forprocessing such a data structure. Examples of time indications are timedurations, time slots and point in time.

Hereinafter, embodiments of the invention will be described in furtherdetail. It should be appreciated, however, that these embodiments maynot be construed as limiting the scope of protection for the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic illustration of a content delivery infrastructureaccording to an embodiment of the invention;

FIG. 2 is a schematic illustration of a head-end system of the contentdelivery structure of FIG. 1 according to an embodiment of theinvention;

FIG. 3 is a diagram illustrating the creation of selection informationfor HTTP Live Streaming according to an embodiment of the inventionusing the head-end system of FIG. 2;

FIG. 4 displays a method for randomizing references to content elements;

FIG. 5 is a schematic illustration of storing content elements in astorage of the head-end system;

FIG. 6 is a schematic illustration of a head-end system according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a content delivery infrastructure 1 forproviding content to a particular recipient device 2A of a plurality ofrecipient devices 2A, 2B over a network 3 according to an embodiment ofthe invention. Network 3 may e.g. represent the internet. The network 3comprises one or more network nodes 4 containing caches for storingdata.

The infrastructure 1 employs a head-end 5 comprising a streaming server6 receiving input from a pre-processing module 7. Streaming server 5comprises a storage (see FIG. 2) for storing content elements CE, aswill be explained in more detail below. The streaming server 6 mayrepresent a system of servers, each of the servers being located near asubset of recipient devices.

The pre-processing module 7 comprises an input for receiving clearcontent and comprises a content modifier CM 8. The output of thepre-processing module 6 is connected to the input of the streamingserver 6.

FIGS. 2 and 3 provide an embodiment of applying the content deliveryinfrastructure of FIG. 1 using HTTP Live Streaming. HTTP Live Streamingbreaks the content stream into a sequence of short content elements,also referred to as chunks herebelow, of equal duration. HTTP LiveStreaming is an example of an adaptive streaming protocol. Adaptivestreaming protocols extend the basic content streaming protocols withsupport for the delivery of content over a network infrastructure thathas no Quality-of-Service guarantees. These protocols extend the conceptof chunk-based transmission by providing multiple copies of the samecontent with different encoding characteristics, such as differentbit-rates and/or different spatial resolutions. Since each quality level(copy) consists of a sequence of time aligned chunks, a recipient devicemay request such a chunk from one of several quality levels at run-timeto react to varying transmission and/or processing conditions.

The architecture diagram of FIG. 2 shows two quality level encoders torender the content at two different encoding characteristics, e.g.bitrates, also referred to as quality levels herebelow. The two chunkgenerators are time synchronised to enable smooth transitions betweencontent chunks of different quality levels. The Playlist Generatorgenerates selection information in the form of a playlist on the basisof information in the chunk store. Since in the adaptive streamingcontent delivery model, the original content chunk is made available inone or more quality levels. This provides a layer of indirection in theplaylist that gives a recipient device the option to select the mostsuitable content chunk. The duration of the chunk rendering is sharedbetween the Synchroniser and the Playlist generator.

It should be noted that in the alternative embodiment the chunk creationmay precede the generation of different quality levels.

Subsequently, one or more of the chunks is modified to generate two ormore modified copies of a chunk using content modifier CM 8 in FIG. 1.This adds a further layer which converts a chunk of a particular qualitylevel into one or more modified content chunks. The modification maycomprise the addition of a watermark or the encryption the chunk, or acombination of these two. It is not necessary that all chunks aremodified, e.g. a watermark may only require a fairly small fraction ofall content chunks to be modified.

The above-described operations may be performed in one or morepre-processing modules 7 of the head-end 5. The one or morepre-processing modules 7 may or may not be part of the streaming server6.

The chunks, including the one or more modified copies the chunks, arethen stored in a storage accessible to the streaming server 6.

The operation of the system is now described in further detail withreference to FIG. 3.

First the original content is processed and the processing result isstored in the storage. To that end, the content is made available at twoquality levels and these are split into chunks in a way that seamlesstransition between chunks of different quality levels are enabled. Inthe embodiment of FIG. 3, the original content (assuming that it isalready encoded) is split into a set of small media files of equalduration. More specifically, the chunk creation produces five mediafiles—chunk₁, chunk₂, chunk₃, chunk₄ and chunk₅. Each chunk has durationof 3 seconds.

The subsequent chunk modification step converts each quality level chunk(e.g. q₁.c₃) into two modified chunks (m₁.q₁.c₃ and m₂.q₁.c₃). Themodification may be the addition of a watermark symbol where two relatedmodified chunks (e.g. m₁.q_(i).c_(j) and m₂.q_(i).c_(j)) contain twodifferent watermark symbols but represent the same content at differentquality levels. In the example of FIG. 3, a sequence of four watermarksymbols is present in the sequence of the four content chunks. As themodified chunks m₁.q_(i).c_(j) and m₂.q_(i).c_(j) contain the samecontent, the recipient device may retrieve only one of these chunks andstill obtain the same content (chunk c_(j) at the desired quality levelq_(i)) but with a different modification (m₁ or m₂).

At the start of the streaming session, the recipient device downloadsselection information in the form of a description file (in the formatof an M3U playlist) that contains one or more ordered lists of media URLreferences to media files (each media file contains a single chunk ofcontent) that the recipient device can fetch. With the content chunksprepared as described above, the streaming server generates theselection information to obtain a unique content file to a specificreceiver. The duration value of a content chunk is defined prior tosegmentation and is stored in the playlist in EXT-X-TARGETDURATION tag.All the streaming content recipient devices use the content URLreferences to obtain various parts of the content. A variant playlist isgenerated to enable the recipient device to retrieve the appropriatequality level. The variant playlist refers to further playlists withdirect references to modified content chunks. By selecting a uniquesequence of modified content chunks, the adaptive streaming system canconstruct a unique content file for a particular streaming contentrecipient device.

For example, the adaptive streaming server 6 in FIG. 1 can prepare aplaylist for each quality level with a unique sequence of modified chunkreferences to insert a unique sequence of watermark symbols. In FIG. 3,the sequence ‘0100’ is inserted by the modified content chunk sequence(m₁ q_(i).c₁, m₂ q_(i).c₂, m₁ q_(i).c₃, m₁ q_(i).c₄). For each qualitylevel, the appropriate water-marking symbol will be generated in theoutput of the recipient device 2A. Even if the recipient device switchesrandomly between instances of the playlist of the first quality leveland the playlist of the second quality level (e.g. as a result ofnetwork transmission times and/or processing conditions), thewatermarking will be performed in accordance with the sequence ‘0100’ aslong as the order of the content chunks c₁, c₂, c₃, c₄ is maintained.Thus, by making a unique sequence of modified content chunks, theadaptive streaming system constructs a unique content file for eachparticular recipient device.

In case the modification is the encryption with a key K₁ or K₂, theDRM/CA system client in the streaming content recipient device issuesthe appropriate key sequence to decrypt the specific sequence ofmodified (encrypted) content chunks. In the example, the key sequence isK₁, K₂, K₁, K₁. Combinations of watermarking and encryption can beaddressed in the same way.

In a variant implementation, the combination of water-marking andencryption is handled by the use of an additional chunk processing layerin the adaptive streaming content delivery system. This would introduceyet another stage in the content preparation process and in theselection information generation.

It should be appreciated that switching between chunks should preferablybe substantially seamless, so the chunks may need to be synchronised toenable seamless switching. Due to the synchronisation, implementationsmay vary the order in which the content preparation processes areperformed, as long as the chunk synchronisation is maintained during theprocessing.

It should further be noted that the requested sequence of quality levelscan also be used for tracking purposes. The downside of this approach isthat the chunk references might be served from a web cache of anintermediate node, which will not be visible to the head-end 5.

The selection information playlists in the example of FIG. 3 has astructured naming sequence for modified content chunks. This may allowan attacker to make educated guesses for alternative content chunks andcreate content in a way that by-passes the tracking mechanism based onwatermarking. Hence, an attacker should preferably be unable to guesswhich modified content chunks contain the same content. This can beachieved by applying a pseudorandom permutation, as illustrated in FIG.4

In the first content preparation step, chunk creation and chunkmodification are performed as described with reference to FIGS. 2 and 3.FIG. 4 depicts this for two different quality settings (q₁ and q₂) andtwo different modifications (m₁ and m₂).

The second step assigns a unique number to each chunk. For instance, achunk with label c_(i)q_(j)m_(k) can be assigned number 4i+2j+k. Itshould be noted that it may then still be straight-forward for anattacker to guess which chunk numbers contain the same content. Forexample, in FIG. 4, chunks 0 through 3 contain the same content indifferent quality levels and with different modifications.

In the third step, each chunk is assigned a (pseudo)random number.Renumbering is done by selecting a pseudorandom permutation σ, andapplying this to the unique number of each chunk. For example, in theabove figure we have that σ(0)=55 and σ(1)=12. In this way, an attackercannot predict which content chunks contain the same piece of content.It is possible to make the permutation unique for each piece of(original) content, by seeding a pseudorandom permutation generator withe.g. the original filename.

Whereas in the embodiment of FIG. 3 the content chunks were storedindividually in accordance with HTTP Live Streaming, the content chunksmay also be stored together in one or more (bigger) files, e.g. a fileof multiple content chunks representing the content for each particularquality level. Individual content chunks may e.g. be identified by astart time, a duration and a file offset as shown in FIG. 5. This mannerof storing the content is e.g. applied by Microsoft Soft Streaming. Thefile format may e.g. be MPEG-4. The Smooth Streaming specificationdefines each content chunk as an MPEG-4 Movie Fragment and stores itwithin a contiguous MPEG-4 file for easy access for each quality level.The content chunks may e.g. be defined as video groups of pictures(GOPs). When a recipient device requests a specific content element froma streaming server applying Microsoft Smooth Streaming, the serverdynamically find the appropriate Movie Fragment with the contiguous fileand send it over the network as a standalone file, such that web cachingis supported. In other words, the content is stored as a single file perquality level, but it is transferred to a recipient device as a seriesof small file chunks.

Storing content chunks in this manner provides for a slightly differentapproach for presenting individual recipient devices with customizedsequences of content chunks while still enabling caching of one or morecontent chunks.

Suppose that a recipient device requests content chunks in the form of aRESTful URL:http://video.foo.com/NBA.ism/QualityLevels(400000)/Fragments(video=610275114) to request content delivery from a server. The valuesin the URL represent encoded bit rate (400000) and the fragment startoffset (610275114) expressed in an agreed-upon time unit (e.g. 100nanoseconds. These values are known from a client manifest obtained fromthe server. As the URL is generated in the recipient device and theparameter fields are linked to the decoding steps of the content, aslightly different mechanism is used to implement the proposedwatermarking scheme. The server issues selection information thatcontains a chunk name lookup table or a reference to a chink name lookuptable. This enables the recipient device to replace the filename(NBA.ism in the above example) for a particular content chunkcharacterised by its start time and duration or end time with the chunkname (e.g. NBA023343.ism) and provide a table that maps timestamps tochunk names. The chunk name lookup table can be individualised by theserver to map the sequence of timestamps to a unique sequence of chunknames for watermarked chunks.

The following examples illustrate the operation of this content deliverymethod.

A recipient device 2A issues a URL:http://video.foo.com/NBA.ism/QualityLevels(400000)/Fragments(video=610275114). The server returns selection information in the formof a chunk name lookup table for a sequence 0110 of watermarkingsymbols:

609275114 NBA182912.ism 610275114 NBA023343.ism 611275114 NBA963713.ism612275114 NBA119992.ismThe chunk name lookup table is then used by the recipient device 2A torequest four content chunks using the following URLs:

1. http://video.foo.com/NBA182912.ism/QualityLevels(400000)/Fragments(video=609275114)

2. http://video.foo.com/NBA023343.ism/QualityLevels(400000)/Fragments(video=610275114)

3. http://video.foo.com/NBA963713.ism/QualityLevels(400000)/Fragments(video=611275114)

4. http://video.foo.com/NBA119992.ism/QualityLevels(400000)/Fragments(video=612275114)

Another recipient device 2B may issue an identical URL:http://video.foo.com/NBA.ism/QualityLevels(400000)/Fragments(video=610275114). The server returns selection information in the formof a chunk name lookup table for a different sequence 0010 ofwatermarking symbols:

609275114 NBA182912.ism 610275114 NBA939921.ism 611275114 NBA963713.ism612275114 NBA119992.ismThe chunk name lookup table is then used by the recipient device 2A torequest four content chunks using the following URLs:

1. http://video.foo.com/NBA182912.ism/QualityLevels(400000)/Fragments(video=609275114)

2. http://video.foo.com/NBA939921.ism/QualityLevels(400000)/Fragments(video=610275114)

3. http://video.foo.com/NBA963713.ism/QualityLevels(400000)/Fragments(video=611275114)

4. http://video.foo.com/NBA119992.ism/QualityLevels(400000)/Fragments(video=612275114)

In the second case the second timeslot references a different chunkname. At the server the chunk name is mapped to a particular contentelement. However, the third and fourth time slot references are commonto the requests for both recipient devices 2A, 2B, such that the serverand one of the recipient devices may benefit from previous caching ofthe third and fourth content elements in an intermediate node in thenetwork 3 of FIG. 1.

Whereas in the above example relating to Microsoft Smooth Streamingquality levels are not applied, it should be appreciated that this wouldsimply add another layer to the example without deviating from theprimary mechanism for defining customizable sequences of content chunksin the server.

FIG. 6 provides an example of an architecture for an embodiment of theinvention wherein quality level differentiation is absent, i.e. adaptivestreaming is not applicable. The operation of this architecture can bethought of as a simplified version of FIG. 3 omitting the quality levelindictor.

One embodiment of the invention may be implemented as a program productfor use with a computer system. The program(s) of the program productdefine functions of the embodiments (including the methods describedherein) and can be contained on a variety of computer-readable storagemedia. Illustrative computer-readable storage media include, but are notlimited to: (i) non-writable storage media (e.g., read-only memorydevices within a computer such as CD-ROM disks readable by a CD-ROMdrive, flash memory, ROM chips or any type of solid-state non-volatilesemiconductor memory) on which information is permanently stored; and(ii) writable storage media (e.g., floppy disks within a diskette driveor hard-disk drive or any type of solid-state random-accesssemiconductor memory) on which alterable information is stored. Thecomputer program may be run on a processor in the head-end 5, preferablyin the streaming server 6.

1. A computer-implemented method for providing content to a particularrecipient device of a plurality of recipient devices, the methodcomprising the steps of: generating copies of one or more contentelements of the content; modifying one or more of the copies of thecontent elements to obtain modified copies of the content elements;storing the content elements, including the one or more modified copiesof the content elements, in a storage; transmitting selectioninformation to the particular recipient device in response to a requestfor providing the content, the selection information prescribing to therecipient device the modified copy to be retrieved by the recipientdevice for substantially each content element for which a modified copyis available.
 2. The method according to claim 1, wherein the step ofmodifying the copies of the one or more content elements comprises thestep of providing different watermarks in different copies of a contentelement and wherein the selection information prescribes the watermarkedcopy to be retrieved by the recipient device for substantially eachcontent element for which a watermarked copy is available.
 3. The methodaccording to claim 1, wherein the step of modifying the copies of one ormore content elements comprises the step of encrypting different copiesof a content element under different key and wherein the selectioninformation prescribes the encrypted copy to be retrieved by therecipient device for substantially each content element for which anencrypted copy is available.
 4. The method according to claim 1, whereinthe copies of the content elements comprise copies for differentencoding characteristics of a content element in order to provide thecontent using an adaptive streaming protocol.
 5. The method according toclaim 1, wherein the step of storing the content elements comprises thestep of storing the content elements in separate files or storing thecontent elements in a single file comprising means for accessing thecontent elements in the file.
 6. The method according to claim 1,wherein the storage storing the content elements is contained in aserver and the method further comprises the step of receiving requestsfor content elements from the recipient device at the server in responseto transmitting the selection information, wherein the number ofrequests for content elements received at the server from the recipientdevice retrieving the contents is less than the number of contentelements stored in the storage.
 7. The method according to claim 1,further comprising the steps of destructuring references to the contentelements in the selection information.
 8. A system for providing contentto a particular recipient device of a plurality of recipient devicesover a network, wherein the system comprises: a copy generatorconfigured for generating copies of one or more content elements of thecontent; a modifying module configured for modifying one or more of thecopies of the content elements to obtain modified copies of the contentelements; a storage configured for storing the content elements,including the one or more modified copies of the content elements; aprocessor configured for generating selection information prescribing tothe recipient device the modified copy to be retrieved by the recipientdevice for substantially each content element for which a modified copyis available; and a transmitter for transmitting the selectioninformation to the particular recipient device in response to a requestfrom the recipient device.
 9. The system according to claim 8, whereinthe modifying module comprises a watermarking module configured forproviding different watermarks in different copies of a content elementand wherein the selection information prescribes the watermarked copy tobe retrieved by the recipient device for substantially each contentelement for which a watermarked copy is available.
 10. The systemaccording to claim 8, wherein the modifying module comprises anencryption module configured for encrypting different copies of acontent element under different key and wherein the selectioninformation prescribes the encrypted copy to be retrieved by therecipient device for substantially each content element for which anencrypted copy is available.
 11. The system according to claim 8,wherein the system comprises a plurality of encoders configured forproviding different encoding characteristics for different copies of atleast one content element and wherein the system is configured toprovide the content using an adaptive streaming protocol.
 12. The systemaccording to claim 8, wherein the storage is configured to store thecontent elements in separate files or as a single file comprising meansfor accessing the content elements in the file.
 13. The system accordingto claim 8, wherein the system is further configured for destructuringreferences to the content elements In the selection information.
 14. Adata structure comprising selection information configured for enforcinga recipient device to retrieve a customized set of modified contentelements of streaming content to be rendered by the recipient device.15. A recipient device configured for processing the data structureaccording to claim 14, wherein the data structure comprises a lookuptable or a reference to a lookup table, the lookup table indicatingdifferent references to the modified content elements for different timeindications and the recipient device is configured for requesting themodified content elements over a network using the references at theindicated time indications using the lookup table.